1. Technical Field
The present invention relates to a printing device, and more particularly, it relates to a printing device comprising an image recording part recording an image on a printing plate on a plate cylinder on the basis of image data.
2. Background Technique
In recent years, a printing device incorporating a prepress apparatus forming an image on a printing plate on the basis of digital image data, the so-called CTP (Computer-To-Plate) apparatus, into the machine is put into practice. Such a printing device is referred to as a digital printing device, and advantageously characterized in that the same is suitable for multi-kind and small quantity printing requiring a short working time or the like since printed matter is directly obtained from image data while even an unskilled operator can readily handle the same since a prepress step or the like is automated.
Such a digital printing device, corrects a mechanical error of a plate cylinder or an encoder is corrected at the time of assembling the printing device. For example, a technique described in U.S. Pat. No. 5,163,368 is well-known as this correction method.
However, the aforementioned prior art prepares a bit memory corresponding to the positions of all dots of an image and stores the position of each dot, and hence the device structure is complicated and cannot be readily performed by an operator for the printing device. Therefore, this prior art has not been in a structure executed by an operator of each printing company every time although the same is executable as adjustment in a manufacturer/maker.
When a printing condition changes, however, when the type of a printing paper or ink, the feed rate for damping water or the like changes, for example, the quantity of displacement of printed matter changes. As a plain example, the quantity of elongation at the time when this printing paper passes through the printing device varies with the type of the printing paper, the thickness or the direction of the paper texture. That is, the printing paper elongates every time the same is brought into pressure contact with each blanket cylinder, and hence the quantity of displacement between a color printed first and a color printed last, for example, changes due to change of this elongation. It has been proven that the quantity of displacement in a plate cylinder rotational direction more remarkably influences the quality of the printed matter than that in the axial direction of the plate cylinder since elongation in the rotational direction of the plate cylinder is particularly large. It has been confirmed that such displacement remarkably changes also when exchanging a blanket of a blanket cylinder.
Registration adjustment responsive to such change of the printing condition cannot be performed by only single adjustment at the time of shipment from the factory dissimilarly to the aforementioned prior art, but adjustment must be performed every operation if necessary. Such adjustment of an image recording position is a troublesome and difficult operation for the operator, and hence an apparatus capable of registration adjustment through the simplest possible procedure has been awaited.
On the other hand, a general multicolor printing device including the aforementioned digital printing machine has such a mechanism that the printing paper is pressed by the number of colors between an impression cylinder and a blanket cylinder to be printed. In such a multicolor printing device, the printing paper tends to slightly elongate every printed color. Therefore, there is such a problem that the positions of a precedently printed image and a subsequently printed image do not match up. It follows that the printing paper is drawn between the impression cylinder and the blanket cylinder while rotating in such a state that the forward end is held in particular, and hence there is such a tendency that elongation on the rear end side of the printing paper is larger than on the forward end side of the printing paper.
In order to solve the aforementioned problem, a conventional multicolor printing device employing a printing plate prepressed through contact image transfer from an original film or the like has employed a technique of changing the position of a prepressed image on the basis of the color of the printing plate.
For example, FIG. 18 is an explanatory diagram showing an example of a case of laying out page images p1 to p8 of eight areas on a printing plate PL. While original arrangement positions of the page images p5 to p8 corresponding to the paper tail side of the printing paper are positions shown by dotted lines in FIG. 18, the same are arranged on the printing plate PL with previous displacement in the direction where the paper elongates as shown by solid lines. When arranging the images with displacement depending on the color plate, color misalignment resulting from elongation of the paper can be reduced. However, the aforementioned technique merely simulatively displaces the positions of the images, and hence correction cannot be made in accuracy as to partial elongation of the images following elongation of the paper.
Further, although it is possible to correct the position every image if it is the layout of a plurality of images such as those described above, fine position control cannot be made every part in a single image arranged substantially over the whole of the printing plate PL. In the case of such a single image, it is conceivable to position the image by changing the magnification for the dimensions of the overall image. However, the quantity of elongation of the printing paper varies with the forward end side and the rear end side, and hence it is impossible to correctly cope with this by such a technique of changing the magnification for the overall image to a constant magnification.
The present invention is directed to a printing device comprising an image recording part capable of recording an image on a printing plate on a plate cylinder and a printing part capable of printing the image on a printing paper with the printing plate.
According to the present invention, the printing device further comprises an input part capable of inputting an evaluation value of a spatial recording error including dimensional change of a printing range caused when printing an objective image on a printing paper in the printing part and a control part controlling the image recording part in response to the evaluation value thereby recording a corrected image subjected to correction of the spatial recording error on the printing plate.
According to the present invention, the spatial recording error including the dimensional change of the printing range resulting from printing is canceled with correction previously applied to the corrected image, so that a correct image is printed.
According to one aspect of the present invention, the evaluation value of the spatial recording error includes a first printing displacement value on an upstream part of the objective image in a rotational direction of the plate cylinder and a second printing displacement value on a downstream part of the objective image in the rotational direction, and the control part comprises a first calculation element calculating a correction value for a recording starting position of the objective image in the rotational direction on the basis of the first printing displacement value, a second calculation element calculating a correction value for an image dimension of the objective image along the rotational direction on the basis of the first and second printing displacement values and a correction value output element supplying the correction value for the recording starting position and the correction value for the image dimension to the image recording part for recording the corrected image on the printing plate.
According to the present invention, the recording position for the image can be readily corrected by inputting the evaluation value including the first and second printing displacement values.
According to another aspect of the present invention, the image recording part is capable of printing a multicolor color image on the printing paper by successively overprinting a plurality of color component images on the printing paper, and the control part controls the image recording part in response to the evaluation value for recording the objective image on said printing plate while stepwisely or continuously changing a recording magnification for the objective image in the axial direction of the plate cylinder along, the rotational direction of the plate cylinder, thereby obtaining the corrected image on the printing plate.
According to still another aspect of the present invention, the image recording part is capable of printing a multicolor color image on the printing paper by successively overlapping a plurality of color component images on the printing paper. The control part comprises a magnification control element controlling the image recording part in response to the evaluation value thereby making the recording magnification for at least one color component image among the plurality of color component images differ from the recording magnification for the remaining color component images, so that area displacement between the plurality of color component images resulting from printing is compensated for by relative difference between the recording magnifications for the plurality of color component images.
Accordingly, an object of the present invention is to provide a printing device capable of readily correcting a recording position for an image even if a dimensional error takes place in printing.
Another object of the present invention is to provide a printing device capable of readily performing registration adjustment of an image in response to change of a printing condition.
Still another object of the present invention is to correct color shift of an image resulting from elongation of a printing paper in a multicolor printing device.